1. Field of the Invention
The present invention relates to a dendrimer solid acid and a polymer electrolyte membrane using the same, and more particularly, to a dendrimer solid acid which provides high ionic conductivity and a polymer electrolyte membrane with excellent ionic conductivity and low methanol crossover.
2. Description of the Related Art
A fuel cell is an electrochemical device which directly transforms chemical energy of oxygen and hydrogen contained in a hydrocarbon material such as methanol, ethanol, or natural gas into electric energy.
Fuel cells can be classified into Phosphoric Acid Fuel Cells (PAFC), Molten Carbonate Fuel Cells (MCFC), Solid Oxide Full Cells (SOFC), Polymer Electrolyte Membrane Fuel Cells (PEMFC), and Alkaline Full Cells (AFC) according to the type of electrolyte used. All fuel cells operate on the same general principles, but may differ in the type of fuel used, the operating temperature, the catalyst used or the electrolyte used. In particular, a PEMFC can be used in small-sized stationary power generation equipment or a transportation system due to its low operating temperature, high output density, rapid start-up, and prompt response to the variation of output demand.
The core part of a PEMFC is a Membrane Electrode Assembly, MEA. An MEA generally comprises a polymer electrolyte membrane and two electrodes on opposite sides of the polymer electrolyte membrane which independently act as a cathode and an anode.
The polymer electrolyte membrane acts as a separator, blocking direct contact between an oxidizing agent and a reducing agent, and electrically insulates the two electrodes while conducting protons. Accordingly, a good polymer electrolyte membrane has high proton conductivity, good electrical insulation, low reactant permeability, excellent thermal, chemical and mechanical stability under normal conditions of fuel cell operation, and a reasonable price.
In order to meet these requirements, various types of polymer electrolyte membranes have been developed, and, in particular, a highly fluorinated polysulfonic acid membrane such as a NAFION™ membrane has been shown to exhibit good durability and performance. However, a NAFION™ membrane should be sufficiently humidified, and to prevent moisture loss, the NAFION™ membrane should be used at a temperature of 80° C. or below. Also, since, a carbon-carbon bond of a main chain is attacked by oxygen (O2), a NAFION™ membrane may not be stable under the operating conditions of a fuel cell.
Moreover, in a Direct Methanol Fuel Cell (DMFC), an aqueous methanol solution is supplied as a fuel to the anode and a portion of unreacted aqueous methanol solution permeates the polymer electrolyte membrane. The methanol solution that permeates the polymer electrolyte membrane causes a swelling phenomenon in an electrolyte membrane and diffuses to a cathode catalyst layer. Such a phenomenon is referred to as “methanol crossover,” and can lead to the direct oxidization of methanol at the cathode where an electrochemical reduction of hydrogen ions and oxygen occurs, and thus the methanol crossover results in a drop in the electric potential of the cathode, thereby causing a significant decline in the performance of the fuel cell.
This same fuel crossover problem may also arise with other fuel cells using a liquid polar organic fuel other than methanol.